Desaturate colors

The
Desaturate command converts a color image to grayscale values, but
leaves the image in the same color mode. For example, it assigns
equal red, green, and blue values to each pixel in an RGB image.
The lightness value of each pixel does not change.

Keep in mind that
the Desaturate command permanently alters the original image information
in the background layer. This command has the same effect as setting
Saturation to ‑100 in the Hue/Saturation adjustment. For nondestructive
editing, use a Hue/Saturation adjustment layer.

Note:

If
you are working with a multilayer image, Desaturate converts the
selected layer only.

Invert colors

The Invert adjustment inverts the colors
in an image. You can use Invert as part of the process of making
an edge mask to apply sharpening and other adjustments to selected
areas of an image.

Note:

Because color print film
contains an orange mask in its base, the Invert adjustment cannot
make accurate positive images from scanned color negatives. Be sure
to use the proper settings for color negatives when scanning film.

When you invert an image, the brightness
value of each pixel in the channels is converted to the inverse
value on the 256‑step color-values scale. For example, a pixel in
a positive image with a value of 255 is changed to 0, and
a pixel with a value of 5 is changed to 250.

Create a two-valued black and white
image

The
Threshold adjustment converts grayscale or color images to high-contrast, black-and-white
images. You can specify a certain level as a threshold. All pixels lighter
than the threshold are converted to white; all pixels darker are
converted to black.

The Properties panel (CC, CS6) or Adjustments panel (CS5)
displays a histogram of the luminance levels of the pixels in the
current selection.

Note:

You can also choose Image > Adjustments
> Threshold. But keep in mind that this method makes direct adjustments
to the image layer and discards image information.

In the Properties panel (CC, CS6) or Adjustments panel
(CS5), drag the slider below the histogram until the threshold level
you want appears. As you drag, the image changes to reflect the
new threshold setting.

Posterize an image

The
Posterize adjustment lets you specify the number of tonal levels
(or brightness values) for each channel in an image and then maps
pixels to the closest matching level. For example, choosing two
tonal levels in an RGB image gives six colors: two for red, two
for green, and two for blue.

This adjustment is useful for
creating special effects, such as large, flat areas in a photograph.
Its effects are most evident when you reduce the number of gray levels
in a grayscale image, but it also produces interesting effects in
color images.

If you want a specific number
of colors in your image, convert the image to grayscale and specify
the number of levels you want. Then convert the image back to the
previous color mode, and replace the various gray tones with the
colors you want.

Do one of the following:

Click the Posterize icon in
the Adjustments panel.

Choose Layer > New Adjustment Layer > Posterize.

Note:

You can also choose Image > Adjustments > Posterize.
But keep in mind that this method makes direct adjustments to the
image layer and discards image information.

In the Properties panel (CC, CS6) or Adjustments panel
(CS5), move the Levels slider or enter the number of tonal levels
you want.

Apply a gradient map to an image

The
Gradient Map adjustment maps the equivalent grayscale range of an
image to the colors of a specified gradient fill. If you specify
a two‑color gradient fill, for example, shadows in the image are
mapped to one of the endpoint colors of the gradient fill, highlights
are mapped to the other endpoint color, and midtones are mapped
to the gradations in between.

You can also choose Image > Adjustments > Gradient
Map. But keep in mind that this method applies the adjustment directly
to the image layer and discards image information.

In the Properties panel (CC, CS6) or Adjustments panel
(CS5), specify the gradient fill you want to use:

To choose from a list of gradient fills,
click the triangle to the right of the gradient fill. Click to select
the desired gradient fill, and then click in a blank area of the
Properties panel (CC, CS6) or Adjustments panel (CS5) to dismiss the
list. For information on customizing the gradient fill list, see Work
with the Preset Manager.

To edit the currently-displayed gradient fill, click
the gradient fill, and then modify the existing gradient fill or
create a gradient fill in the Gradient Editor. (See Create
a smooth gradient.)

By default, the shadows, midtones, and highlights of the
image are mapped respectively to the starting (left) color, midpoint,
and ending (right) color of the gradient fill.

Select either, none, or both of the Gradient options:

Dither

Adds random noise to smooth the appearance of the gradient
fill and reduces banding effects.

Reverse

Switches the direction of the gradient fill, reversing
the gradient map.